In crystals that exhibit birefringence, the birefringence is generally a function of wavelength, and narrow-band optical filters have been disclosed based on a change of sign of birefringence which occurs at a predetermined wavelength in certain uniaxial semiconductor crystals such as ZnO, CdS, CdS.sub.1-x Se.sub.x and Zn.sub.1-x Cd.sub.x S. The crystal is disposed between either crossed or parallel polarizers, and coupling of light energy between ordinary and extraordinary polarizations at the predetermined wavelength is effected by applying a magnetic field or stress to the crystal. Further details concerning optical filters to the foregoing type may be found in the papers, C. H. Henry, "Coupling of Electromagnetic Waves in Cds", Physical Review, Vol. 143, No. 2 (March 1966), pages 627-633; J. P. Laurenti et al, "Optical Filters Using Coupled Light Waves in Mixed Crystals", Applied Physics Letters, Vol. 28, No. 4 (Feb. 15, 1976), pages 212-213; and J. P. Laurenti et al, "Graded-Composition Semiconductors as Tunable Narrow-Band Optical Filters", Journal of Applied Physics, Vol. 48, No. 1 (January 1977), pages 203-204.
Electronically tunable optical filters have been provided wherein a spatially periodic dc electric field applied to a birefringent medium disposed between appropriately oriented polarizers is utilized to electronically program the filter with a wide variety of transmission characteristics. This filter relies upon a type of electro-optic effect that produces an effective optic axis rotation in the birefringent medium while the magnitude of the birefringence remains constant over the entire wavelength range of interest. For further details concerning this type of electronically tunable filter, reference may be made to patent application Ser. No. 864,356, filed Dec. 27, 1977, by D. A. Pinnow et al, "Electro-Optic Tunable Optical Filter", and assinged to the assignee of the present invention, and to the paper by D. A. Pinnow et al, "An Electro-Optic Tunable Filter", Applied Physics Letters, Vol. 34, No. 6 (Mar. 15, 1979), pages 391-393.